1. Field of the Invention
The present invention relates to semiconductor electronic circuit structures that selectively employ gates on two sides of device channel regions to reduce the area necessary for the circuit structures. The invention is particularly suited for use in a static random access memory (SRAM) of a very dense form and is applicable to nano-scale CMOS technology.
2. Description of the Background Art
The SRAM is the most common form of fast memory in digital electronic circuits. Indeed, for most common special, general-purpose and graphics processors, SRAMs occupy nearly half or more of the chip area. Use of fast local memories continues to increase as a fraction of the areas because it is the fastest way of improving performance currently. There are some specialized applications where the dynamic random access memories supplant SRAM when density is more important than speed. However, by and large, SRAMs are the preferred form of memory in systems-on-chip, in most logic applications, and in many levels of cache memory that enclose the processor.
The SRAM's most common form, however, uses 6 transistors that take up a large area (nearly 10 times the logic gate area). As an example, a planar device SRAM cell usually consists of 18 via-contacts to devices, 2 contacts to supplies and a total of 14 interconnects just within the cell. Thus, it usually takes nearly 80-100 squares of lithographic features even with common contacts in the cell-arrays. This is what causes an SRAM to have a large cell size. As devices have shrunk in dimension, there is increased use of silicon on insulator structures in order to improve the electrostatics. As devices shrink further in the sub-100 nm range, the expectation is that the technology will evolve to the use of very thin silicon films, double gates or some variations thereof, as well as three-dimensionally integrated structures are expected. The density of an SRAM is a major constraint in density and speed. A systems-on-chip (SOC) approach allows one to integrate SRAMs into a variety of applications beyond those of digital processors. Dense SRAMs, thus, have a major application range across the spectrum of most applications of electronics.